The panel fell off window seven, damaging tiles on a "carrier panel" on the left Orbital Manoeuvring System (Oms) pod.

The Oms pod houses an engine and propellant for the shuttle to manoeuvre in orbit. The carrier panel seals the Oms pod to the rest of the orbiter.

"When we discovered the damage, we went ahead and pulled a spare carrier panel. The Oms pods are interchangeable, so the carrier panels are as well," said Ms Stilson.

"We found that it fitted perfectly."

The repair procedure takes about an hour to complete.

The soft window cover which fell is made of plastic with a foam edge and is taped in place to protect the windows and keep them clean before launch.

'Written in blood'

Earlier, Nasa administrator Dr Mike Griffin had given a frank press conference outlining the importance of launching the shuttle.

"There is no recovery from the mistakes we've made, whether it's going back to the Apollo fire, the loss of Challenger, or the loss of Columbia; going back to go through 100 years of flight, the lessons that we who fly know are written in other people's blood," he told reporters at Nasa's Kennedy Space Center in Florida.

"Obviously, it is utterly crucial for Nasa, for the nation, for our space programme, to fly a safe mission," Dr Griffin added.

"We have done everything that we know to do."

Its mission is scheduled to last 12 days, with landing set for 1101 EDT (1601 BST; 1501 GMT) on 25 July at the Kennedy Space Center.

The mission will carry spare parts and other equipment to the International Space Station.

Columbia and its crew of seven perished as they flew home for landing, the result of undetected damage from a debris impact during lift-off that had no effect on the ship in space.

One final obstacle to Wednesday's launch may be stormy weather forecast for the area.